1. Field of the Invention
The present invention relates to a method to determine the effective production parameters for a UV cationic polymerizable coating system.
2. Discussion of the Background
Photo polymerization technology is conventionally employed for the coating of substrates including, for example, paper, furniture and vinyl flooring. This technology is advantageous in comparison to other known coating methodologies because of lower cost and significantly reduced environmental concern since volatile solvents are not employed.
Of the available UV polymerizable systems, free radical polymerization has until recently, been the technology of choice in applications such as offset printing, lithographic printing and metal coating. However, UV polymerization suffers several disadvantages which must be addressed in industrial applications. The polymerization process depends upon generation and reaction of free radical units which are short-lived and sensitive to destruction by free radical scavengers such as oxygen. Therefore, the system to be coated must be made inert to prevent oxygen inhibition. In addition, due to the extremely short lifetime of free radicals, polymerization via free-radical addition proceeds only as a result of UV irradiation. When irradiation ceases, free radical polymerization quickly terminates.
When coating complex three dimensional substrates, such as an automobile body part, the requirements for successfully applying a free radical polymerizable coating, i.e., application in an oxidation inert environment and full and complete irradiation of the entire substrate surface including areas shaded from the light source prohibit the use of or render free radical polymerization coating processes uneconomical.
Cationic photopolymerization is an alternative technology to free radical polymerization which has several unique advantageous properties especially to facilitate its use in the coating of complex three dimensional substrates. Cationic polymerization proceeds via cationic active centers which are not sensitive to free radical scavengers such as oxygen. Therefore, application in an oxidation inert environment is not necessary. Secondly, in contrast to free radical light-induced polymerizations which experience rapid termination via radical exchange with any local scavenger when illumination ceases, the cationic active centers are non-terminating. They do not combine with one another, and termination occurs predominantly through relatively slow processes such as combination with counter ions or active center entrapment due to polymerization of the surrounding monomer. As a consequence of this non-terminating property, the cationic active centers have extremely long lifetimes which allows the polymerization to proceed post-illumination until monomer is consumed or the reactive center is entrapped in the polymer matrix.
The effects of the post-illumination polymerization inherent in cationic photopolymerization is several fold. Post-illumination the cationic active centers are mobile and migrate throughout the coating, thus polymerizing available monomer. This phenomenon leads to the occurrence of a polymerization front which migrates by a process of reactive center diffusion in all available directions of non-polymerized monomer composition. The continuation of polymerization leads to “dark” polymerization where the polymer matrix continues to form in the absence of light and into a depth of the coating which was not reached by the UV illumination. Thus, cationic photopolymerization may be employed to cure thick systems, i.e., coatings greater than 1 mm in depth. In addition to curing in the depth direction of the coating system, polymerization also proceeds in directions lateral to an axis defining the depth direction. This leads to “shadow” curing where even surface areas shaded from illumination polymerize and cure.
For the coating of complex substrates with cationic curable systems, in an industrial environment, an accurate method including a fundamental model which describes active center generation and both dark and shadow cure may be of great value. Such a method would allow characterization and understanding of the effects of a variety of process variables with a minimum number of experimental or pilot runs, and thus save set-up costs and improve production efficiency.
U.S. Pat. No. 6,833,154 describes a process for radiant curing of a coating on a three dimensional object. The process includes (a) providing a model of a radiant output of at least two lamps to be used to provide the radiant curing of the coating; (b) providing a model of at least one characteristic of a response of the coating to radiant curing; (c) selecting radiant output of the lamps based upon the model of the radiant output; and defining at least a spatial position of the lamps during curing of the coating; (e) defining at least a spatial position of the three dimensional object during curing of the coating; (f) simulating a radiant output of the at least two lamps based on the defined spatial position of the lamps; (g) in response to the simulated radiant output and the defined spatial position of the three-dimensional object during curing of the coating, determining if a predicted radiant output of the at least two lamps on the three-dimensional object will acceptably cure the three dimensional object; and (h) if the radiant output is acceptable, storing a number of the lamps, position of the lamps used in the model of the radiant output, the selected radiant output, and the defined position of the three-dimensional object.
U.S. Pat. No. 6,544,334 describes systems and methods for creating a combinatorial coating library including a coating system operatively coupled to at least one of a plurity of materials suitable for forming at least one coating layer on a surface of one or more substrates. The systems and methods also include a curing system operative to apply at least one of a plurality of curing environments to each of a plurality of regions associated with the at least one coating layer, the curing system comprising a plurality of waveguides each having a first end corresponding to at least one of a plurality of regions and a second end associated with at least one curing source. The combinatorial coating library comprising a predetermined combination of at least one of the plurality of materials and at least one of the plurality of curing environments associated with each of the plurality of regions.
U.S. Pat. No. 6,490,501 describes a monitoring and control system for use in curing composite materials including a model for a workpiece being cured. The model calculates current internal states of the workpiece and predicts, based upon past and current states of the workpiece, future states of the cure process. These future states are represented as virtual inputs to the controller, which controls operation of the cure process based upon both real and virtual inputs. Cure rates are affected by both external temperatures and internal heat generated by the curing process itself. The internally generated heat is considered by the model when calculating current states and predicting future states. By projecting the cure state into the future, problems caused by high cure rates can be avoided. In addition, pressure can be optimally controlled in response to estimated internal material state.
U.S. Pat. No. 5,182,056 describes a stereolithographic apparatus and a method for generating a part from curable material. The invention utilizes control and/or knowledge of depth of penetration of actinic radiation into a vat of photopolymer to determine and/or control and/or produce desirable characteristics associated with the creation of parts. From a predictive point of view, such characteristics may include determination of cure depth from a given exposure, determination of cure width, determination of required minimum surface angle, determination of optimum skin fill spacing, the strength of cross sections of partially polymerized material, amount of curl type distortion and necessary overcure to attain adhesion between layers. These determinations can lead to the use of particular building techniques to insure adequate part formation. From the controlling and producing point of view, the penetration depths can be controlled to obtain optimized for a given layer thickness, maximized speed of drawing, minimized print through, maximized strength, minimum curl and other distortions, and maximum resolution, etc. Resin characteristics may be integrated with the depth of penetration associated with the particular resin being used and the wavelength(s) of actinic radiation being used to solidify it, and with the intensity profile of the beam of actinic radiation as it strikes the resin surface.
U.S. Patent Application Publication 2004/0148051 describes a modeling method for minimizing the cure time and flow time of a thermoset in-mold coating for a molded article. The minimization of cure time is based on determining the cure time as a function of mold temperature and initiator level. The method for minimizing flow time is based on predicting the fill pattern for a specific geometry mold. The methods may be used to reduce cycle time in the preparation of in-mold coated parts and reduce defects in the appearance of the coating.
However, none of these references are directed to U.V. curable cationic polymerization system and none consider migration of cationic active centers post illumination.
U.S. Patent Application Publication 2008/0305273 describes a method of applying a polymer coating to a complex three dimensional substrate. The substrate, such as a automotive vehicle body, is coated with a combination of a coating formulation and a photoactivated mixture containing active centers that have been produced prior to application. The two liquids can be intimately mixed prior to application to the object, or the coating formulation can be applied prior to the application of the photoactivated mixture. The coating formulation is cured by the active centers that have been produced prior to application.
However, this reference does not describe a method for determining or predicting the optimum process parameters which provide efficient and effective curing of the coating.